Stable dielectric fluid for electrical discharge machining comprising a mineral oil, a metal sulfonate and a phenolic antioxidant

ABSTRACT

Dielectric fluids are provided for electrical discharge machines comprising a primarily aliphatic light oil of low viscosity having in minor amounts an oil soluble metal sulfonate detergent and a phenolic antioxidant.

United States Patent Stayner [54] STABLE DIELECTRIC FLUID FOR ELECTRICALDISCHARGE MACHINING COMPRISING A MINERAL OIL, A METAL SULFONATE AND APIIENOLIC ANTIOXIDANT [72] Inventor: Robert A. Stayner, Palos VerdesEstates,

Calif.

[73] Assignee: Chevron Research Company, San Francisco, Calif.

[22] Filed: June 10, 1968 211 Appl. No.: 735,578

[ 1 Feb. 1, 1972 2,485,861 10/1949 Campbell et al ..252/l8 2,501,7323/1950 Mertes .2$2/32.7 2,672,448 3/1954 Newnan ...252/64 2,813,83011/1957 Trautman ..252/63.7 2,943,052 6/1960 Carlyle et al....,....252/63.5 3,168,638 2/1965 Riddles ..2l9/69 3,249,539 5/1966Detweiler et al... 252/334 3,390,247 6/1968 Webb ..219/69 PrimaryExaminerHarold Ansher Assistant Examiner-D. J. Fritsch Attorney-A. L.Snow, F. E. Johnston, John Stoner, Jr. and B. I. Rowland [57] ABSTRACTDielectric fluids are provided for electrical discharge machinescomprising a primarily aliphatic light oil of low viscosity having inminor amounts an oil soluble metal sulfonate detergent and a phenolicantioxidant.

5 Claims, No Drawings STABLE DIELECTRIC FLUID FOR ELECTRICAL DISCHARGEMACHINING COMPRISING A MINERAL OIL, A METAL SULFONATE AND A PI-IENOLICANTIOXIDANT BACKGROUND OF THE INVENTION Field of the InventionElectrical discharge machining is particularly useful for precisionmachining. The economics of electrical discharge machining are governedto a great extent by the wear rate of the electrode and the rate ofcutting of the workpiece. The dielectric fluids used can effect changesin both of these.

The'dielectric'fluid acts as a carrier of the charge from the electrodeto the workpiece. Furthermore, it serves to remove detritus whichresults from the vaporization and cooling of the metal which evaporatesfrom the surface of the workpiece due to the thermal action of thesparks. In practice, the dielectric fluid is recycled, being collectedfrom the apparatus, filtered by a convenient filtration means such as adiatomaceous earth and then returned to the apparatus. The dielectricfluid also serves to cool the workpiece, so that the crystal structureof the body of the workpiece remains substantially the same.

Satisfactory fluids should have controlled dielectric properties so asto provide a sizable charge flowing from the electrode to the workpiece,should be of a light color so that the workpiece can be observed throughthe oil, should be able to carry detritus to the filter for filtration,and should be oxidatively and thermally stable for long periods of time.

SUMMARY OF THE INVENTION Compositions are provided for electricaldischarge machining which have excellent stability, remain transparentfor long periods of time, prevent deposition of detritus anddepositforming organic material, and maintain a useful dielectricstrength, the compositions comprising an oil of lubricating viscosityhaving a minor aromatic content, an alkaline earth organic sulfonate anda phenolic antioxidant.

DESCRIPTION OF THE PREFERRED EMBODIMENTS The compositions of thisinvention are primarily comprised of a gas oil, an alkaline earth metalorganic sulfonate detergent, an antioxidant and optionally minor amountsof other additives. The first material to be considered will be the baseoil.

Gas oil is a general term originally applied to a petroleum distillateboiling between kerosene and lubricating oils. Kalichevsky-Kobe,Petroleum Refining With Chemicals, Elsevier Pub. Co., New York, (1956),page 23. The gas oils which are preferred for use in this invention arelightly acid treated and boil from about 400 to 600 F.

The flash point of the oil will generally be at least 200 F. andpreferably at least 2l5 F., usually not exceeding 300 F. (COC). Theviscosity at l F. will be fairly low, usually at least about 30 SSU andnot exceeding about 50 SSU. The aniline point will be at least 100 F.and usually will be in excess of I00 P. not exceeding 160 F. Ordinarily,there will be from about to 30 weight percent of aromatics in thecomposition, the remaining composition being aliphatic and naphthenic.The oil is substantially free of aliphatic unsaturation 5 weight percentolefins).

The organic sulfonate which is added is an oil soluble alkaline earthmetal organic sulfonate. The alkaline earth metals which find use arecalcium and barium, calcium being preferred. The organic sulfonateswhich find use may be neutral (stoichiometric equivalence of metal andacid) or have as much as 0.5 moles excess of the metal as basic metal,e.g., hydroxide or carbonate. Usually, the organic sulfonate will besubstantially neutral.

Any of the alkaline earth metal organic sulfonates which find use aslubricating oil detergents may be used. Descriptions of the varioussulfonates may be found in U.S. Pat. Nos. 2,485,861, 2,50l,732, and2,270,577. The sulfonic acids may be derived from natural petroleumsources and are referred to as the mahogany sulfonates. Other sulfonicacids may be prepared from alkyl substituted benzenes having alkylsubstituents of from about eight to 30 carbon atoms. The sulfonatedetergent will generally be of at least l5 carbon atoms and usually notgreater than 75 carbon atoms, more usually of from about 20 to 50 carbonatoms.

The particular method or the particular sulfonic acid which is used toprepare the alkaline earth metal organic sulfonate is not critical tothe use of the composition in this invention.

The amount of sulfonate which will be used will be sufficient to providefrom about five to 30 p.p.m. in the final fluid. The amount of thematerial should not reduce the dielectric strength of the finalcomposition below 8 kv. The dielectric strength of the electricaldischarge machine fluid will generally be in the range of about 10 to 40kv., more usually of from about 12 to 35 kv. During thecourse of use ofthe electrical discharge machining fluid, some of the sulfonate may bedepleted with a concomitant increase in the dielectric strength of thefluid.

The final additive is the antioxidant which is a phenolic compoundhaving the following formula:

R R l I wherein X is a bond, methylene, isopropylidene, or sulfur and Ris a tert.-alkyl group of from four to five carbon atoms, i.e.,tert.-butyl and tert.-amyl.

The antioxidants which find use in this invention are, therefore,bis-phenols having terL-alkyl groups in the ortho-positions andpara-attachment between the phenolic groups.

The antioxidant will generally be added in about 0.005 to about 0.1weight percent, more usually of from about 0.008 to about 0.05 weightpercent.

Various other additives may also be included, such as masking agents andodorants to provide a pleasant fragrance and hide the characteristichydrocarbon odor of the base oil which may be objectionable.

The compositions of this invention may be readily prepared by merelymixing the ingredients at an elevated temperature, usually of from aboutto F. and stirring until homogeneous.

The following composition was prepared when 100 parts of gas oil havingthe following properties was combined with 0.005 weight percent ofcalcium mahogany sulfonate having 1.67 weight percent calcium, preparedby the sulfonation of 480 neutral oil and neutralization with calcium,and 0.0l percent of the methylene (bis-[2,6-di-(tert.-butyl)phenol]).

Also included was an odorizer"(Deodall No. l) in an amount of 0.005weight percent. The final product has the following; properties:

TABLE II Gravity, All ASTM D-287 36.7 Flash point. F.- ASTM D-92 220Aniline point, "P. ASTM D-ll2 I45 Viscosity at l00 F.. SSU ASTM D-2l6l34.0

Suli'onates, p.p.m. Dielectric strength. kv. ASTM color This compositionwas used commercially with electrical discharge machines over a period 3months, where only makeup oil was used to replenish oil lostmechanically. The oil stayed substantially transparent over this timeand provided significant improvement over the oil which had beenpreviously used. The work time was reduced; the wear of electrodes wasdiminished, and the reuse of the oil was greatly extended.

As will be evident to those skilled in the art, various modifications onthis invention can be made or followed, in the light of the foregoingdisclosure and discussion, without departing from the spirit or scope ofthe disclosure or from the scope of the following claims.

I claim:

1. Dielectric fluid for electrical discharge machining consistingessentially of Y i a mineral oil having a flash point of at least 200F., a

viscosity at 100 F. in the range of 30 to 50 SSU, and from about to 30weight percent aromatics and substantially free of aliphaticunsaturation;

from about 5 to 30 p.p.m. of an oil soluble alkaline earth metal organicsulfonate; and,

from 0.005 to about 0.1 weight percent of a phenolic compound of theformula:

wherein X is a bond, methylene,isopropylidene or sulfur and R is atert.-alkyl group of from four to five carbon atoms, and wherein thefinal composition has a dielectric strength in the range of 8 to 40 kv.

2. A composition according to claim I wherein said phenolic compound is(bis-2,6-di-(tert.-butyl)phenol).

3. A composition according to claim 1 wherein said alkaline earth metalsulfonate is calcium mahogany sulfonate.

4.- A composition according to claim 3 wherein from 0 to 0.5 mole excessof calcium base is present.

5. A composition according to claim 1 wherein said mineral oil has from10 to 30 weight percent aromatics and less than 5 weight percentolefins.

2. A composition according to claim 1 wherein said phenolic compound is(bis-2,6-di-(tert.-butyl)phenol).
 3. A composition according to claim 1wherein said alkaline earth metal sulfonate is calcium mahoganysulfonate.
 4. A composition according to claim 3 wherein from 0 to 0.5mole excess of calcium base is present.
 5. A composition according toclaim 1 wherein said mineral oil has from 10 to 30 weight percentaromatics and less than 5 weight percent olefins.